The invention relates to the field of streak tubes and more particularly to circular scan streak tubes.
Circular scan streak tube (CSST) systems have been proposed for detecting, recording and temporally resolving optical events on the order of picoseconds. Much of the investigation in this field has been conducted as part of developing a spaceborne laser ranging system. Ranging applications, however, may be conducted with a pair of laser pulses, or a periodic series of pulses. In the latter case, CSST receiving equipment can be synchronized with the outgoing emissions to allow detecting low-level returning signals by summing (integrating) the resulting optical event signals. Work in the ranging application, accordingly, has not adequately addressed the detection, storage and readout of random data, particularly over record lengths longer than a single streak tube scan period.
A fundamental limitation of streak tube data readout is that optical events occur and can be recorded as data faster than the data can be read out electronically. This limitation constrains real time recording of optical events to a brief interval triggered by the event to be recorded. Additionally, triggering recordal from the event itself inherently misses initial data, so it becomes necessary to continuously record and store data to save initial information.
Circular scan streak tubes overwrite or sum the stored data during each successive scan, so storage is limited to only the number of pixels (m=2.pi.) in a single circumference of the target. Merely increasing the size of the target and number of pixels is subject to practical limitations. One is the economics of making large arrays. Another limitation is that lateral deviations of the swept electron beam must be kept small in high-time-resolution devices to minimize differences in transit time between peripheral and axial electrons.
Several possible kinds of readouts for circular scan streak tubes have been considered. The Photochron IIC streak tube as reported in 1984 included a phosphor screen at the output of the tube, covered by a fiber optic faceplate. The streaked output images were recorded photographically on film. See W. Sibbett et al, "Photochron IIC streak tube for 300 MHz circular-scan operation," SPIE Vol. 491 High Speed Photography (Strasbourg 1984). This method provides a data record length limited to a single scan.
Later, improved signal detection was reported in W. Sibbett and W. E. Sleat, "A Photochron IIC Circular-scan Streak Camera with CCD Readout", SPIE Vol. 674 High Speed Photography (Pretoria 1986), pp 543-558 and A. Finch, et al, "Electron-sensitive CCD Readout Array for a Circular-scan Streak Tube," SPIE Vol. 59l Solid State Imagers and Their Applications (1985), pp 31-37. Both of these papers describe the Photochron IIC as a system including a circular array target comprising an array of photodiodes as sensing elements and a CCD shift register to read out the resulting charge. While sensitivity was improved, this arrangement is no less limited in record length to data collected over a single circular scan of the electron beam. After one scan, it is necessary to blank the writing beam, or deflect it off of the electron-beam-sensitive portion of the target to avoid overwriting.
C. B. Johnson, et al, "Circular-scan streak tube with solid-state readout", Vol. 19, No. 20 Applied Optics (Oct. 15, 1980) describes a circular-scan streak tube (CSST) having a circular photodiode array as a sensing element. The array is fiber-optically coupled to the output phosphor screen of the tube. The array readout circuitry is gated or triggered responsive to first (start) and second (stop) laser pulses to record first and second streaked output signals, respectively, on the readout array for measuring the time between the laser pulses. The acquired data thus is single-shot rather than continuous.
A commercial streak camera and readout system is the Hadland 2DR system, described in D. L. Bowley, "Measuring Ultrafast Pulses,"Lasers & Optronics, Sept. 1987, pp 81-83. The Hadland 2DR target employs a rectangular area CCD array, fiber-optically coupled to the camera or experiment. It apparently operates in a single-scan, triggered mode.
It would be desirable to provide for a circular scan streak tube target capable of detecting and storing event signal data over a longer period of time, preferably several times the period of the electron beam scan.
Accordingly, the need remains for a circular scan streak tube target capable of providing a continuous stream of data responsive to a continuously scanning electron beam, so that data is captured and available for analysis that corresponds to a period of time before and after a triggering event.